I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to fast forward link power control in a code division multiple access system.
II. Description of the Related Art
Multiple access techniques are efficient techniques for utilizing the limited radio frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
Frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using narrowband signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M. K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. A particular receiver then determines which signal is intended for that receiver by a unique spreading code in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
Since multiple radiotelephones and base stations transmit on the same frequency, power control is an important component of the CDMA modulation technique. A higher power output by a radiotelephone and/or base station increases the interference experienced by the other radiotelephones and base stations in the system. In order to keep the radiotelephones and base stations from transmitting at too much power, thereby lowering system capacity, some form of power control must be implemented.
The radiotelephone can aid the base station in the control of the power on the forward link (from the base station to the radiotelephone) by transmitting a power control message to the base station on the reverse link (from the radiotelephone to the base station). The radiotelephone gathers statistics of its error performance and informs the base station via a power control message. The base station may then adjust its power level to the specific user accordingly.
In a typical CDMA cellular communication system that follows the Telecommunications Industries Association/Electronic Industries Association Interim Standard 95 (IS-95), the base station adjusts its forward link power at a rate no faster than once per frame. This results in a high required .sup.E b/N.sub.o on the forward link during low speed travel of the radiotelephone, due to the effects of Rayleigh fading, since the radiotelephone remains in the fade longer.
The ratio .sup.E b/N.sub.o is a standard quality measurement for digital communications system performance. The ratio expresses the bit-energy-to-noise-density of the received signal. .sup.E b/N.sub.o can be considered a metric that characterizes the performance of one communication system over another; the smaller the required .sup.E b/N.sub.o the more efficient is the system modulation and detection process for a given probability of error. A more detailed discussion of this concept can be seen in B. Sklar, Digital Communications, Fundamentals and Applications, Chapter 3 (1988).
Data frames communicated between base stations and a mobile radiotelephone are divided into groups of consecutive coded bits where each group is referred to as a power control group. In IS-95 CDMA, the power control groups are 1.25 ms long. The length of the power control group is typically equal to the frame length divided by the number of power control updates per frame.
This power control scheme estimates the received .sup.E b/N.sub.o over a power control group of 1.25 ms for the 800 Hz power control updates. The estimate is compared against an .sup.E b/N.sub.o threshold referred to as the (.sup.E b/N.sub.o).sub.set point. Up or down power control bits are generated by the mobile radiotelephone as a result of the comparison. The (.sup.E b/N.sub.o).sub.set point is updated after each frame is decoded.
The (.sup.E b/N.sub.o).sub.set point is increased by a predetermined amount, Step_Up, if the frame just decoded was in error and decreased by another predetermined amount, Step_Down, if the frame was received correctly. Step_Up and Step_Down are related as follows: ##EQU1##
where Target_FER is the frame error rate (FER) that the power control process is trying to achieve.
The power control bits are transmitted to the base station where the power is adjusted according to the value of the power control bits. The power control bits to control the power on the forward link are sent on a separate control channel. Therefore, unlike the control of the reverse link power, where power control bits are punctured on the forward link traffic channel, the forward link power control command bits are not punctured on the reverse link traffic channel.
To estimate the .sup.E b/N.sub.o, the received signal power and the received interference need to be estimated. Since the data on the pilot channel is known, the received interference can be easily estimated from the pilot channel. Once an estimate of the interference is available, the estimate of the received signal power must be made.
The estimate of the signal power over a power control group may be made from the traffic channel if the data rate of the current frame is known. However, in a variable rate voice call, the data rate is not known for the frame until the frame has been decoded. A previously unknown need exists to determine a reliable estimate of the signal power for a traffic channel.